Currently marketed internal combustion engines are monitored cyclically to determine combustion characteristics (e.g. engine knock or misfiring) and so adjust combustion parameters (e.g. spark lead), and also to detect any faults and indicate the need for repair. Monitoring is normally based on signals from sensors inside the engine. For example, engine knock is normally determined using a signal from an accelerometer fixed to the crankcase or cylinder head of the engine.
Using sensors installed inside the engine is fairly expensive in terms of purchase cost and installation of the sensors, and the wiring required to connect the sensors physically to an electronic control unit.
Patent Application EP1843024A1 proposes determining the intensity of sound pressure waves generated by an internal combustion engine using a sound pressure sensor (i.e. a microphone), and determining the value of at least one engine operating parameter accordingly. In other words, Patent Application EP1843024A1 proposes replacing traditional sensors, installed inside the engine, with a microphone for determining the sound pressure waves generated by the engine, and which can be integrated directly in the electronic control unit with no further wiring needed.
Patent Application EP1843024A1, however, proposes processing the microphone signal in the same way as a conventional sensor (e.g. accelerometer) signal, without taking into account the characteristics of the sound pressure waves generated by the engine. As a result, the microphone signal analysis findings are invariably inaccurate.